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Hurricane FLORENCE


Hurricane Florence Discussion Number  43
NWS National Hurricane Center Miami FL       AL062018
1100 PM AST Sun Sep 09 2018

Florence's satellite appearance has continued to improve quite
markedly since the previous advisory. An eye was evident in GOES-16
high-resolution infrared imagery and other channels between
2300-0000 UTC, but it became cloud covered immediately thereafter
due to a strong burst of deep convection in the southern and eastern
eyewall where cloud tops colder than -80C and an abundance of
lightning activity was observed. Since that time, the CDO has
expanded and become more circular, outflow has increased and become
more symmetrical, and an eye has begun to re-appear. The initial
intensity has been increased to 80 kt for this advisory based on a
subjective Dvorak intensity estimate of T4.5/77 kt from SAB and NHC
objective intensity estimates ranging from 77 kt to 87 kt.

The initial motion estimate is 280/06 kt. Once again, there is no
significant change to the previous forecast track or synoptic
reasoning. The models appear to be getting better dialed in on both
the location and strength of the developing blocking ridge in the
vicinity of Bermuda during the next 4 days as the mid-latitude flow
amplifies across the CONUS and the northwestern Atlantic. In fact,
the global models are now in very good agreement on forecasting
Florence's upper-level mass outflow being deposited to the north and
east of the hurricane, which will act to further strengthen the
blocking ridge and help to drive the hurricane northwestward toward
the southeastern U.S. coastal region. The new NHC model guidance is
even more tightly packed about the previous forecast track, with
less than a 90 n mi cross-track spread at 72 h and less than 120 n
mi spread at 96 h, just prior to expected landfall. Therefore, the
new official advisory track is essentially just an update and
extension of the previous forecast, and lies between the nearly
juxtaposed HCCA and FSSE corrected consensus track models.

Now that Florence has developed an inner-core ring of deep
convection, which has insulated the eye from intrusions of dry air,
rapid intensification appears likely to begin soon and continue for
the next 36 hours or so due to the expected very low vertical wind
shear conditions, dual outflow jet pattern that will be developing,
and very warm SSTs of 29-29.5 deg C beneath the hurricane. The most
favorable combination of the aforementioned factors will occur in
about 48 h, and that's when Florence is likely to achieve its
maximum intensity.  After 72 hours, the wind shear is expected to
increase to around 10-15 kt from the south or southwest, and the
dual outflow pattern is forecast to change to only a single poleward
outflow pattern. This slight degradation in the upper-level
environment, along with slightly cooler SSTs, is expected to result
in a gradual weakening of the powerful cyclone. However, Florence is
still forecast to be near category-4 strength when the dangerous
hurricane makes landfall. The official intensity forecast is an
average of the intensity forecast from the corrected consensus
models HCCA and FSSE, with the latter explicitly forecasting a peak
intensity of 134 kt in 72 hours. It is also worth noting that the
model guidance is also significantly increasing the size of
Florence's wind field over the next few days, and the official
forecast reflects this trend.

The NOAA G-IV jet will conduct another synoptic surveillance
mission early Monday morning in support of the 1200 UTC model
cycle, and these flights will continue through Tuesday. In addition,
upper-air stations across portions of the central and eastern U.S.
are conducting special 0600 UTC and 1800 UTC radiosonde launches to
collect extra data for the numerical models. Hopefully these data
will help improve the track and intensity forecasts.

Key Messages:

1. There is an increasing risk of two life-threatening impacts from
Florence: storm surge at the coast and freshwater flooding from a
prolonged heavy rainfall event inland. While it is too soon to
determine the exact timing, location, and magnitude of these
impacts, interests at the coast and inland from South Carolina into
the mid-Atlantic region should closely monitor the progress of
Florence, ensure they have their hurricane plan in place, and follow
any advice given by local officials.

2. Large swells affecting Bermuda and portions of the U.S. East
Coast will continue this week.  These swells will result in
life-threatening surf and rip currents.


INIT  10/0300Z 24.6N  57.7W   80 KT  90 MPH
 12H  10/1200Z 24.9N  59.1W   90 KT 105 MPH
 24H  11/0000Z 25.6N  61.5W  105 KT 120 MPH
 36H  11/1200Z 26.5N  64.4W  120 KT 140 MPH
 48H  12/0000Z 27.8N  67.6W  130 KT 150 MPH
 72H  13/0000Z 31.2N  73.6W  130 KT 150 MPH
 96H  14/0000Z 34.0N  77.5W  120 KT 140 MPH
120H  15/0000Z 35.2N  79.0W   55 KT  65 MPH...INLAND

Forecaster Stewart